The snowy white branching coral front-and-center of this image is severely bleached, while the boulder corals around it have experienced minimal bleaching.Photo: ARC Centre of Excellence for Coral Reef Studies/ Gergely Torda

Back-to-back bleaching events in 2016 and 2017 killed one in every two corals on the Great Barrier Reef. Now, looking just at that first year of heat-inducted death, a team of Australian researchers has concluded that the character of the northern Great Barrier Reef has been forever altered.

The study, published today in Nature, places the blame squarely on rising global temperatures driven by human carbon emissions.

“Scientists have been saying for a long time that bleaching events will become more frequent and change the mix of species,” lead author Terry Hughes of the ARC Centre of Excellence for Coral Reef Studies told Earther. “What our paper shows is that transition is already well underway.”

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Bleaching—when corals eject the symbiotic algae that live in their tissue, turn a pasty white, and begin to starve—occurs when temperatures rise just a little above corals’ comfort zone. When the water stays overheated for weeks on end, corals start to die due to heat stress and hunger.

That’s exactly what happened on the Great Barrier Reef in 2016, when a heat wave descended on the northern half of the 1,400 mile-long ecosystem. Aerial photos revealing vast swaths of colorless, seemingly lifeless reef went viral, introducing the world to one of the first casualties of the Anthropocene. Scientists continued to study the event as it unfolded, eventually concluding that 30 percent of the corals were killed that year. They also found that climate change made the marine heat wave up to 175 times more likely, concluding that the “bleaching would be almost impossible without climate change.”

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Those same scientists have been doggedly tracking the Great Barrier Reef’s health ever since, and have now put together a detailed snapshot of the ecological transformation that took place from March to November 2016.

“We went back to the same reefs eight months after the bleaching, basically to see what was left,” Hughes told Earther.

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This section of the Great Barrier Reef highlights that certain species (white ones) are more affected by bleaching events than others. Photo: ARC Centre of Excellence for Coral Reef Studies/ Mia Hoogenboom

By matching satellite-based information on heat exposure to maps of coral death, Hughes and his colleagues learned that coral mortality began after about three to four weeks of overheating. After six weeks in the slow cooker, more than half of corals on a given reef were typically dead, and the mix of species left alive was radically transformed.

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Staghorn and tabular corals, which give a reef its “nooks and crannies, the 3D habitat that fish and other creatures depend on” as Hughes put it, were particularly devastated.

“The change in the communities was rapid and resulted in the loss of most of the fast-growing corals, which also provide much of the reef structure,” study co-author Mark Eakin of NOAA’s Coral Reef Watch program told Earther in an email.

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Overall, nearly 30 percent of the 3,800 individual reefs comprising the Great Barrier Reef were transformed in 2016 “from mature and diverse assemblages to a highly altered, degraded system,” according to the new study. Most of those reefs are in the northern third of the ecosystem.

“Depressing stuff,” Kim Cobb, a coral reef scientist at the Georgia Institute of Technology who wasn’t involved with the study, told Earther in an email. “The paper lays out in gory detail how much damage a single marine heat wave can [wreak] on key ecosystems.”

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We’re just starting to learn what sorts of ecological ripple effects this transformation will have. Hughes says researchers are already seeing a decline in reef fish, especially species that eat corals, or those that live inside in their nooks and crannies. Most reef fish at least depend on corals as a nursery, meaning the recent heat wave could have widespread impacts on breeding success that affects commercial fisheries down the line.

Coral reefs can recover from mass mortality events given enough time. But time is not a luxury climate change seems to be affording. In 2017, despite the absence of an El Niño event to drive ocean temperatures up, the Great Barrier Reef experienced another mass bleaching. That time around, the central section of the reef was hit hardest, with another 20 percent of all corals killed, according to Hughes.

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“We’re concerned about the shrinking gap between pairs of bleaching events,” Hughes said, noting that in the 1980s, mass bleaching events occurred on average about once every 25 years.

That gap has since narrowed to once every five to six years, with water temperatures often hotter during cool La Niña years today than they were during warm El Niño years a few decades ago.

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If we can stabilize global temperatures soon, says Hughes, the Great Barrier Reef will have a fighting chance. He’s currently leading research on recoverability—determining which sections of the reef are most resilient so that they can be protected to reseed the depauperate reefs of the future.

“You can describe the Great Barrier Reef glass as being half full,” Hughes said. “We still have half the corals and they’re tough. They’ve gone through one hell of a natural selection event.”

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Still, if we can’t limit global warming to something close to the 2 degree goal of the Paris climate agreement, all bets are off for the Great Barrier Reef’s future.

At four to six degrees of global warming, Hughes said, “there won’t be any corals.”